The development of power electronic devices with improved performance, increased reliability, compacted size, and reduced weight requires the passive components, such as resistors, capacitors, inductors, and transformers, to be stacked or embedded within a printed wire board (PWB). This technology could free up surface space, increase device reliability, and minimize electromagnetic interference and inductance loss. The capacitance density of a dielectric is proportional to its relative permittivity (∈r) divided by the thickness of the dielectric material. A high performance capacitor can be constructed using a dielectric with sufficient permittivity.
A high temperature capable capacitor can be fabricated by using ceramic film dielectric of high permittivity, such as perovskite ceramics having the general formula ABO3. Thin ceramic films can be deposited on base metal foils, such as nickel and copper, to form film-on-foil sheets. These film-on-foils exhibit not only high relative permittivity (∈r≧1000) and low leakage current density (Jleakage<1×10−8 A/cm2) but also high dielectric breakdown strength (Eb≧2×106 V/cm). Energy that can be stored inside a capacitor is proportional to the square of applied voltage. It is highly desirable to have capacitors with high breakdown strength for power electronic applications, such as for use in electric drive vehicles.
Fabrication of capacitors, containing at least two electrical conductors separated by a dielectric, is affected by material and manufacturing limitations. Normal irregularities in the raw material or fabrication processes produce minor flaws or weaknesses within the capacitor structure. Typical variations in the composition or thickness of the electrodes may result in slight variations in the formation of the electrical field. Likewise, slight variations in the composition of the dielectric may result in differences in the localized physical and electrical properties within the dielectric. Further, fabrication processes for formation of the dielectric layer on the conductor may produce voids, inclusions or micro cracks, which again may result in differences in the localized composition and properties within the dielectric. When exposed to electric fields near the operating limits of the device these irregularities may concentrate the electric field over time to result in a breakdown of the device.
A capacitor of two electrodes 2 separated by dielectric 4 is shown in the schematic drawing FIG. 1. When the capacitor is subjected to an external electrical potential, electric dipoles inside the dielectric 4 align in response to the applied electric field, resulting in net displacement charge accumulation near the electrodes 2. The effective field inside a dielectric material under external field is smaller than the applied field. When a ceramic dielectric material 4 is subjected to a substantially high electric field, dielectric breakdown can originate from defects inside the dielectric material or as a result of high electrical stresses. The dielectric breaks down when the current rises sharply at a critical electric field; permanent damage is often found along fine tubular channels or cracks, while the major portion of the sample is left intact. Similar to an applied mechanical load which causes mechanical fracture, an electric field can cause dielectric breakdown. The breakdown process is connected to an initiating microcrack, void, inclusion, tubular micro-flaw, or other types of defects that can trap substantial amounts of static charges to produce a local electric field exceeding a threshold value. This leads to the propagation of material failure. A defect-free sample breaks down at a field specific to the material, independent of the sample. This solid state phenomenon has been attributed to a few electrons in the conduction band, accelerated by the applied field, liberating more valance or trapped electrons.
A micro-crack initiated failure, as shown in FIG. 2 is used as example for discussion. The formation and propagation of crack 6 is strongly related to the electric field (or charge concentration) at the tip of the crack. During aging process (i.e. under a low applied field) the local electric field at the tip 8 can be reduced by the injected space charge (field limiting space charge). As a result, the crack propagates slowly from one electrode to the other one. On the other hand, during dielectric breakdown test (i.e. when the applied voltage is continuously increasing) and under special conditions (i.e. a little field moderation in particular regions of the sample), the energy for the formation and the propagation of the filamentary crack may be quickly reached, leading to catastrophic breakdown. In ferroelectric materials, structural defects related to electrostriction (inverse piezoelectric effect) can potentially cause aggregated dielectric breakdown.
Breakdown of capacitors is a performance-limiting property in circuits. The measured breakdown strength is sensitive to defects, electrodes (both material and geometry), and environment.
U.S. Pat. No. 7,436,650 discloses a laminated ceramic capacitor with a stress relieving layer formed between capacitance layers to provide high breakdown voltage. The stress relieving layer, ceramic dielectric layers, dummy inner electrode layers (split electrodes) that do not contribute to the formation of electrostatic capacitance, and capacitance-formation-preventing inner electrode layers that prevent capacitance from being formed between the capacitance-forming inner electrode layers and the dummy inner electrode layers are laminated. The stress relieving layers increase the capacitor's breakdown strength.
A need exists in the art for capacitors with high breakdown strength for power electronic applications. The capacitor should be designed so as to avoid catastrophic breakdown within the ceramic dielectric films.